Manufacture of lithium grease



z- 8, 1956 P. J. BAKER 2,760,936

MANUFACTURE OF LITHIUM GREASE Filed May 20, 1952 2 Sheets-Sheet l r TOEVACUATION EQUIPME NT SOAP an. IE

STORAGE INVENTOR. PETE. R J. BAKE-R Aug. 28, 1956 P. J. BAKERMANUFACTURE OF LITHIUM GREASE Filed May 20, 1952 PENETRATION A.S.T.M.PENETRATION ASIM.

2 Sheets-Sheet 2 aeo'F z-Io'F aeov= a9o|= sooF 5\or= BLEND\NGTEMDERATURE FIE-7- 550 o 50 roo \50 200 50 500mm BLENDER SHAFT SPEEDINVENTOR.

PETER J. BAKER United States Patent MANUFACTURE OF LITHIUM GREASE PeterJ. Baker, Louisville, Ky., assignor, by mesne assignments, to NationalCylinder Gas Company, Chicago, 11]., a corporation of DelawareApplication May 20, 1952, Serial No. 288,854

3 Claims. (Cl. 252-41) This invention relates generally to themanufacture of grease compositions and more particularly to a novelcontinuous process for forming improved grease compositions of lithiumsoaps and oils.

It is well established that the quality of a grease varies substantiallywith the method of making it, and also that a satisfactory process forforming a grease of one metal soap and oil is not necessarilysatisfactory, or even operative, for another metal soap even though thesame oil be used. This is believed to be due to the fact that differenttypes and lengths of fibers or micelle structures are formed bydilferent soaps, and by the same soap when different processes offorming the fibers are utilized. Moreover, the degree of orientation andtwist of the fibers is a variable from grease to grease and affects thephysical properties thereof.

Grease fiber or micelle formation is essentially a process ofcrystallization which is brought about by cooling a solution of soapfrom a relatively high temperature to a lower temperature at which thesoap is insoluble in the oil. Various factors including rate of coolingand the degree of agitation during cooling through the criticalcrystallization range and thereafter affect the type of fibers which areformed. After the fibers have been formed the structure may be modifiedby milling which tends to minimize sineresis and improve the stabilityof the grease.

The heretofore prevalent process for manufacturing lithium greaseinvolves the following steps: Initially a slurry of lithium soap inmineral oil is prepared in open kettles and heated to a temperature ofabout 400 F. to completely dissolve the soap. The soap may be preparedseparately or in the slurry kettles from fats or fatty acids and lithiumhydroxide in the presence of part or all of the oil. If only part of theoil is used to form the soap the remainder of the oil is added aftersaponification is completed and the slurry is heated to solutiontemperature. The solution of lithium soap in oil is then drawn from thekettle and allowed to cool statically, usually in open pans, until a gelis formed. The cooled gel is then subjected to a milling operation inwhich it is worked to form a homogeneous grease composition. Thisprocesshas a number of practical disadvantages. The high temperatureheating in open kettles results in loss of more volatile components fromthe oil and creates a fire hazard. The process involves considerablemanual handling of pans of gelled grease both in filling and feedingthegelled grease into the mill, which is time consuming and involves adanger of contamination. For these and other reasons, the process cannotbe controlled to produce uniform greases from batch to batch, and, evenmore important, the yield of grease whish may be obtained from a givenamount of soap is low. It is necessary to deaerate the grease during themilling operation. Furthermore from the standpoint of economy ofoperation, the power required to mill the cooled grease is quite high,and the .heat requirement to raise the slurry including Patented Aug.28, 1956 the entire amount of oil in the grease to solution temperatureis considerable.

I am aware that other processes have been proposed for the continuousmanufacture of grease which in some respects more nearly resemble my newprocess than does the above described pan cooling method. Most of theseother processes, however, are not specific to greases made from lithiumsoap and although they may work out satisfactorily for other metal soapgreases they have not proved satisfactory for lithium greases, asevidenced by the continued use of the laborious pan cooling process bylithium grease producers.

I have provided an improved process for producing lithium grease whichmay be operated on a continuous or semi-continuous basis and whichproduces higher yields of grease from given amounts of soap (the mostexpensive constituent) and also produces grease of more uniformcharacteristics and superior quality. Moreover, my new process resultsin economics in labor, electric power and heat required.

One of the important features of my invention is the discovery that theabove mentioned improvement may be obtained by blending a hot solutionof the soap in part of the oil with additional oil having asubstantially lower temperature, such blending being accomplished withina certain critical temperature range and under conditions of extremelymild agitation conducive to complete mixing with low shear. Forpractical purposes the critical temperature range lies between about 230F. and 310 F. Following this rapid shock chilling by blending, theresultant greaseis additionally cooled under conditions of violentagitation to a temperature below a second critical temperature belowwhich the soap micelle is stable and which I have found to be about 160F. The grease may then be finished by milling in the conventionalmanner. In connection with the milling operation, I have found thatconsiderably less power is required to mill the grease than in the caseof pan cooled greases. this is not fully understood, but itis believedto be due to the fact that the rapid chilling without substantialagitation .during the blending step produces more soap micelle or fibersof the desired size and a lesser number of fibers of excessive length sothat substantially all of the fibers are of the desired size and lessmilling power is required to break down overly long fibers.

This factor of proper fiber growth initially in the cooling stepis alsobelieved to account for the higher yields experienced with my processsince the fibers which are formed are utilized with greatereffectiveness in the gel to produce adequate penetration values orstiffness in the finished grease even though the soap is present insmaller amount. Obviously if a pan .cooled grease must be worked toreduce extra. long fibers, that same working reduces the shorter fibersto less than optimum gel forming length.

Another particular advantage of my process is that virtually no gelforming time is involved. Gelation is completed by the time the greaseis cooled below the second critical temperature, and consequently noholding vessels are required.

Other objects and advantages of the improved process of my inventionwill become apparent to those familiar with the art on reading thefollowing specification in connection with the drawings and the appendedclaims. In the drawings:

Fig. 1 is a flow diagram showing a preferred system of apparatus forcarrying out my novel process;

Fig. 2 is a longitudinal vertical section showing somewhat schematicallythe internal construction of a preferred form of heat exchange unit;

Fig. 3 is a transverse section of such unit, taken on line 3-3 of Fig.2;

The reason for Fig. 4 's a longitudinal vertical section showingsomeschematically the internal construction of a preferred form ofblender;

Fig. 5 is a transverse section of the blender taken on line 55 of Fig.4;

Fig. 6 is a graph showing the effect of blending temperature onpenetration; and

Fig. 7 is a graph showing the effect of blender shaft speed onpenetration.

As previously indicated my improved process may be operated eithercontinuously or semi-continuously. For continuous operation a pair ofidentical slurry tanks 10 are preferably employed. These tanks areprovided with conventional agitators to keep the solids in suspensionand are connected through a suitable line 11 to evacuating equipment(not shown) so that the slurry may be deaerated at this stage ratherthan in the finishing mill. Oil is supplied to the slurry tanks from theoil storage container 9 through conduit 12, either by gravity feed or apump (not shown). prepared and added in solid form.

From the tanks 10, which are discharged alternately, the slurry isconducted by a valved conduit 13 to a slurry pump 14 which forces itthrough the heat exchange and blending apparatus described below. Theoutlet of the pump 14 is connected by a conduit 15 to the slurry heatingunit 16 where the slurry is agitated and heated to a solutiontemperature within the range of about 375 F. to about 450 F., dependingupon the particular soap used.

The construction of heating unit 16 is preferably identieal to that ofthe cooling unit 17, so that Figs. 2 and 3 are illustrative of either ofthese two heat exchangers. Each unit comprises a pair of generallycircular end plates 20 and 21 recessed to receive the ends of the heattransfer tube 22. Ring members 23 are sleeved on the tube 22 adjacent tothe end plates and support-a pair of cylindrical elements 24 and 25.Thus an annular space 26 is provided for heat transfer fluid around thetube 22. The space between the sleeves 24 and is preferably filled withinsulation 27 to provide a heat insulating jacket.

The end plates 2i and 21 are provided with appropriate bearings andseals for journalling the agitator shaft 28 and sealing the point atwhich the shaft passes through the plate 21. The agitator shaft 28 fillsa major portion of the space within the tube 22, and together with thetube forms a thin elongated annular passage 29 for the grease or slurry,as the case may be. The agitator shaft carries a plurality of blades orscrapers 30 which serve to prevent sticking of material to the innersurface of heat transfer tube 22 and to agitate the grease or slurry asthe shaft rotates at relatively high speed, i. e. on the order of 500 R.P. M. A suitable drive mechanism (not shown) is provided to rotate theshaft at this speed.

In the case of the slurry being heated in the unit 16 the violentagitation aids in providing uniform heating and solution of the soap,while in the case of the grease being cooled in the cooling unit 17 theagitation serves to control micelle fiber growth and gel formation bypromoting rapid controlled cooling from the critical blendingtemperature to the lower critical milling temperature.

The slurry or grease, as the case may he, enters the heat exchangerthrough an inlet conduit 31 at one end and leaves through an outletconduit 32 at the opposite end. The heat transfer fluid is conducted toand from the annular space 26 by appropriate connections 33 and 34.

To those familiar with the art of heat transfer it will be readilyapparent that the units 16 and 17 effect very rapid and efiicientheating andcooling of the slurry and grease under closely controlledconditions of temperature, pressure and degree of agitation. The heattransfer fluid in the case of the unit 16 is preferably a heating pmedium having a boiling point at atmospheric pressure much higher thanthat of water, such as diphenyl or diphenyl oxide or a mixture of thetwo sold under the trade name Dowtherm. Cold water or brine may be .usedin the cooling unit 17.

The soap is preferably previously Intermediate the two heat exchangeunits 16 and 17 a blender 35 is provided. A preferred form of blender isillustrated in Figs. 4 and 5. r This unit is similar in construction tothe two heat exchange units in that an annular passage for the productis provided between a hollow cylinder 51 and a rotating shaft 52. Theends of the cylinder 51 are supported by end plates 53 and 54 whichcontain suitable bearings for the ends of the shaft 52. A seal 55 isprovided in the plate 54 to prevent product leakage at the drive end ofthe shaft. Suitable means (not shown) are provided to drive the shaft ata slow speed to provide gentle agitation. Thorough mixing with low shearis accomplished by a series of pins 56 arranged in a helical pattern onthe shaft 53 which cooperate with a row of similar stationary pins 57mounted in the wall of the cylinder 51. The product enters and leavesthe blender through inlet and outlet connections 58 and 59,respectively, provided in the end plates 53 and 54.

The purpose of the blender, as will be explained later in more detail,is to mix the hot solution emerging from heating unit 16 with relativelycold oil. The blender alternatively may be in the form of a length ofpipe having several bends therein or of a chamber having circuitouspassages therethrough. The use of a slowly driven mixing shaft in theblender is preferred, however, because it imposes less load on the pumps14 and 37 than does a static system. In the illustrated embodiment theoil is pumped from the tank 9 through a conduit 36 by means of an oilpump 37. The oil pump and the slurry pump are preferably interconnectedin any suitable manner to proportion the relative amounts of slurry andcold oil which are supplied to the blender. The ratio of oil in theslurry to cold oil is preferably about 1:1 although other ratios withinthe range of 3:1 and 1:3 have been tried and proved successful. Incertain cases the oil from which the slurry is formed and the cold oilsubsequently added may be of different types, in which cases separateoil storage tanks are used.

An oil heater 38 is shown in the drawing. This heater is not essentialto the operation of the process, but its use is preferred for itprovides a very simple arrangement for controlling the critical blendingtemperature. The heater 38 may be a conventional heat exchanger to whichsteam is supplied through a line 40. The steam line 40 is preferablyrovided with an automatic control valve 42 which is connected asillustrated at 43 so as to be responsive to the temperature of thegrease emerging from the blender 35 in the conduit 41 leading to thecooler 17. Thus the critical blending temperature is controlled bysupplying more or less heat to the relative cold oil prior to its beingmixed with the hot solution in the blender 35. The output temperature ofthe oil heater 38 is usually maintained within the range of F. to 180F., but actually this temperature is dependent uponthe oil-slurry ratioand the blending and solution temperatures and may be variedconsiderably either above or below this range. For example, if a largeproportion of the oil is initially incorporated in the slurry in slurrytank 10, it may be necessary to refrigerate the cold oil to reduce theslurry from solution temperature to the desired blending temperature. Inthis latter situation the apparatus 38 takes the form of a refrigeratingdevice rather than a heater.

The critical blending temperature may also be controlled by varying therelative amounts of oil and slurry or by varying the temperature towhich the slurry is heated. The control of relative rates of oil flow,however, has proved much more diflicult than the control of steamsupplied to a heat exchanger such as indicated at 38.

From the blender 35 thegrease, at the critical blending temperature andin a gutty state indicating fiber or gel formation, is supplied to thecooling unit 17. in this unit its temperature is quickly lowered undercontrolled agitation in the thin annular space 29 to below the criticalmilling temperature of about F. At this temperature gel formation isbelieved to be complete and the grease is ready for milling andpackaging. It is desirable not to exceed the critical millingtemperature in the milling operation, and, if the type mill employedproduces considerable temperature rise, it is preferred to chill thegrease to a somewhat lower temperature in the unit 17. The millingoperation, which may be performed in a suitable milling device indicatedat 45, differs from the conventional procedure in that deaeration is notrequired and in that considerably less milling power is needed asdescribed. Experiment has shown that suflicient power is saved in themill to more than ofiset the mechanical power used in the two heatexchangeunits 16 and 17 and in the blender 35. Thus an overall economyin power is achieved by my process.

I have found that the blending temperature has very important bearing onfinal quality and characteristics of the grease, and that the blendingtemperature of each grease is quite critical. The effect'of the blendingtemperature upon the penetration is shown graphically in Fig. 4.Penetration is a measure of hardness of grease and indirectly a measureof the yield which may be obtained from a given soap content, the lowerthe penetration and the harder the grease and the better the yield.Curve X represents a grease formed of a lithium soap and a mineral oilhaving a viscosity index of 60. Curve Y represents a grease formed of asimilar soap having a slightly lower softening point and a differentoil, having a viscosity index of 41.8. In each case the soap content was7%. As will be apparent from Fig. 4, the optimum blending temperature isabout 270 F. in the first case and about 290 F. in the second. It hasbeen found that different oils and soaps and percentages of soap producesimilar curves but dilferent critical blending temperatures. Althoughthe exact relationships are un predictable, nevertheless the criticaltemperatures may be readily determined by simple and conventionalpenetration test procedures. In general, however, the critical blendingtemperature has been found to be between about 230 F. and 310 F.

I also have found that the degree of agitation during blending is quiteimportant. In Fig. 7 the relationship between agitation during blendingand penetration is illustrated. In obtaining this curve a shaft typeblender such as is illustrated in Figs. 4 and 5 was used and the speedof the shaft 52 was varied. The penetration of the greases correspondingto various shaft speeds was measured. The product cylinder 51 of theblender 35 used in this test had a diameter of three inches and'was 12inches in length. The cylinder 51 has a single row of stationary pins 57spaced an inch part. The shaft 52 was two inches in diameter and had aseries of pins which extended through the shaft to protrude on eachside. The shaft pins 53 were spaced about an inch apart along the shaftaxis and were arranged in a helix. As will be seen from the curve ofFig. 7 the optimum shaft speed is about 60 R. P. M. in this blender 35.With this speed there is obtained a complete mixing of solution and coldoil and rapid cooling of the soap solution to critical blendingtemperature. The gentle agitation produces little shear and there arevirtually no disruptive forces developed which might cause micelle fiberbreakdown. For purposes of the process herein described, the extent ofagitation employed at this point shouldbe a. minimum consistent withthorough mixing of the hot solution with the cold oil. Any greaterdegree of agitation involves excessive shear producing a softer greaseand decreasing the possible yield. This explains the equivalency of alength 'of pipe or labyrinth to the blender 'unit 35.

A very important advantage of my process is the fact that the total heatrequirement is substantially reduced since a large amount of the oil isnot heated to solution temperature, as in the conventional processherein before referred to, but is introduced relatively cold. This isimportant from an operating as well as an initial equipment coststandpoint. For example, where about 50% of the oil is introduced coldthe capacity of the heat. source supplying heating medium to the unit 16need be only half as great as if all of the oil had to be heated as inthe conventional process.

As stated previously it is preferred to use a ratio of 1:1 between coldoil and oil in the slurry. When this ratio is'maintained, it is possibleto use heating and cooling units of the same size. The heating unit 16handles only one-half the flow of the cooling unit 17 due to the factthat the cold oil does not pass through the heating unit. However, thetemperature rise in the heating unit is about twice as much as thetemperature drop in the cooling unit with the result that units havingthe same Example A Fourteen pounds of .prepared lithium stearate knownas Witco #306, having a softening point 'of 210 C. and manufactured bythe 'Witco Chemical Company, 295 Madison Avenue, New-York, New York,were mixed with eighty-six pounds of a naphthenic petroleum oil, havinga pour'point of 15 F., a flash point of 410 F., a viscosity of 650 SSUat100 F. and 60.6 SSU at 210 F., and a viscosity index'of 41.8, to forma'slurry containing 14% lithium soap. This slurry was then pumpedthrough a heating unit similar to unit 16 shown in Figs. 1-3 at a rateof approximately pounds per hour. The shaft speed in the unit was 500 R.P. M., and the temperature of the slurry was raised to about 425 F. Theresultant hot solution was blended with one hundred pounds of the sameoil having a temperature of about 160 F. by pumping the oil and hotsolution through a 10 foot length of /2 inch pipe having eight rightangle bends to form grease at a temperature of approximately 290 F. Thecold oil was supplied at a rate of about 75 pounds per hour so thecombined flow through the pipe was about 150 pounds per hour. Pumpingthe hot solution and cold oil through the pipe produced a gentleagitation equivalent to that provided in the rotating shaft blenderdescribed. above when the shaft is rotated at very low speeds to providerapid chilling of the hot solution with very low shear. Immediatelyafter blending the grease was passed through an agitated cooling 'unit,similar to that hereinbefore described and having an agitator shaftspeed of ,500 R. P. M.,

to lower its temperature to below 150 F. The rate of flow through thecooling unit was about 150 pounds per hour. Next the grease was workedin a Charlotte colloidv mill (Model W10).

. .The finished grease had a soap content of 7% and a penetration (ASTM)of 300 after 60 strokes in a conven-' tional grease worker. It exhibitedno bleed upon standing and stood up well on additional working, showinga difference of penetration of only 33 after 10,000 strokes in thegrease worker.

In order to determine the effect of blending temperature on the yield asmeasured by penetration, a series of greases was prepared by theprocedure of the above example with all conditions maintained'the sameexcept that the cold oil temperature was varied to give a series ofblending temperatures from about 274 F. to about 307 F. The 60 strokepenetrations of the resultant greases were measured and the results whenplotted gave the curve Y illustrated in Fig. 6. The grease having thelowest penetration value was obtained with a blending temperature ofabout 290 F.

Example B Fourteen pounds of a prepared lithium stearate known as Witco#305 and similar to the soap in Example A, but having a slightly lowersoftening point, namely 200 C., were added to eighty-six pounds ofmineral oil having a flash point of 380 F., a pour point of 0 F., aviscosity of 567 SSU at F., and a viscosity index of 60. The oil andsoap were mixed cold to form a slurry having 14% soap. A stream ofsuchslurry was continuously withdrawn and heated to 425 F. by the procedureof Example A to dissolve the soap. and then was blended with a stream ofcold oil having various temperatures ranging from 120 F. to 160 F. inthe blender 35 described in connection with Fig. 7. The two streams weresupplied to the blender 35 at equivalent rates so that equal volumes ofhot solution and cold oil were mixed. This produced blendingtemperatures ranging from 262 F. to 285 F. The blender shaft speed wasabout 60 R. P. M. The penetration values'of the resultant greases, whichwere cooled and finished as in Example A, were measured and plotted toproduce curve X of Fig. 4. As shown by this curve the optimum blendingtemperature for maximum yield occurs in the range of 2681F. to 275 F.for the particular oil and soap utilized. All of the resultant greaseshad good texture and appearance and were not subject to sineresis.

Example C A slurry was prepared from the soap described in Example A andthe oil referred to in Example B. This slurry contained 25% soap.Portions of this slurry were heated to approximately 425 F. and thenwere separately blended with sutficient cold oil inv the mixing blenderdescribed in connection with Fig. 7 to produce greases having soapcontents of 7%, 9.5% and 11%, respectively. The blender shaft speed was60 R. P. M., and the blending temperature. in each case was held ataround 270 F. by adjusting the cold oil temperature which was variedbetween about 145 F. and about 210 F. Afterblending, the greases werecooled and milled as in Example A.

Although somewhat more viscous than the slurries of the precedingexamples which had lower soap contents, the 25% slurry was easily pumpedthrough the system by virtue of the fact that the rotary blenderwasemployed. No significant diiferences in appearance, texture or stabilitybetween greases made by my process'from 14% slurries and from 25%slurries were noted, and all of the greases of this example were freefrom syneresis. The penetration values of the three greases of thisexample were as follows:

Penetration (ASTM) After Working in Grease Worker Example D Lithiumhydroxide and stearic acid in combining proportions were mixed in a gasfired heating kettle with two thousand pounds of mineral oil to form asolution containing 14% soap after saponification was completed. Thetemperature of the mixture was raised to 400 F. to elfect saponificationof the acid and hydroxide and complete solution of the soap produced.The mineral oil used had a viscosity of 500 SSU at 100 F, Two thousandpounds of cold oil having a temperature of about 140 F. were rapidly runinto the kettle to lower the temperature of the batch to about 270 F.During the foregoing steps the batch was stirred gently by conventionalpaddle agitators rotating at 30 R. P. M.

The resulting grease was then run through a chilling unit similar to theunit 17 and its temperature was rapidly lowered to about 120 F. Thethroughput rate was approximately 1800 pounds per hour and the agitatorshaft speed was 400 R. P. M. The cooled grease was milled in acentrifugal grease worker and packaged.

The resultant grease was free from syneresis and exhibited goodstiffness both before and after additional working in aconventionalgrease worker. The 60 stroke penetration (ASTM) was 234 andthe penetration after 5000 strokes was 275.

From the foregoing specific examples, which are included for purposes ofillustration and not by way of limitation, it will be apparent that myprocess may be carried out with various different types of apparatus andmay also be varied to utilize different oils, soaps and slurrycompositions.

Various other changes and modifications, in addition to those set forthherein, may be made without departing from the spirit of my inventionthe scope of which is commensurate with the following claims.

What is claimed is:

l. The process of preparing lithium grease comprising forming aslurry oflithium stearate in a portion of the mineral oil desired in the finishedgrease, continuously passing a stream of said slurry through a heatingzone and heating the slurry in said zone to a temperature at Which thelithium stearate is completely soluble in the oil and above about 400 F.while subjecting the slurry to violent agitation sufficient to produce asubstantial shearing action, thereby to effect thorough and intimatemixing of the lithium stearate and oil and form a homogeneous solutionof the lithium stearate in the oil free from undissolved lithiumstearate particles, adding the remainder of the oil desired in thefinished grease to the solution under conditions of substantiallyshear-free gentle agita tion, the added oil having a temperatureselected with regard to the proportionate amount of the oil added inthis step. to produce a resultant temperature of the mixed oil andsolution of about 280 F., passing the resultant mixture through acooling zone under conditions of violent agitation sufiicient to producesubstantial shearing action-and rapidly cooling said mixture in saidzone to a temperature below F., and finally milling the Cooled productfor a sutficient time to eliminate syneresis and provide stabilityin thefinished grease.

2. The process of preparing lithium grease comprising forming a slurryof lithium stearate and mineral oil containing substantially all thelithium stearate and approximately half the oil desired in the finishedgrease, raising the temperature of the slurry to a temperatureconsistent with complete solution of the lithium stearate in the oil andabove about 400 F. to completely dissolve the lithium stearate in theoil and form a homogeneous solution free from undissolved lithiumstearate particles by passing the slurry in a thin layer under agitationin contact with a heat transfer wall, continuously mixing the remainderof the oil desired in the finished grease with the hot solution underconditions conducive to mixing with a minimum of shear to provideintimate mixing without micelle disruption, the added oil having atemperature such as to produce a resultant temperature of the mixed oiland solution within the range from about 230 F. to 310 F., passing theresultant mixture through a cooling zone under conditions of agitationto reduce the temperature thereof below 160 F., and finally milling theproduct for a sulficient time at a temperature below 160 F. to eliminatesyneresis and produce stability in the finished grease.

3. The process of preparing lithium grease comprising forming a slurryof lithium stearate and mineral oil containing substantially all thelithium stearate and a portion of the oil desired inthe finished grease,raising the temperature of the slurry to a temperature consistent withcomplete solution of the lithium stearate in the oil and above about400F. to completely dissolve the lithium stearate in the oil, adding theremainder of the oil desired ill the finished grease under mildsubstantially shear-free agitation, the added oil having a temperaturecloser to room temperature than to the solution temperature and selectedwith regard to the proportionate amount of the oil added in this step toproduce a resultant temperature of the mixed oil and slurry of betweenabout 230 F. and 310 F., passing the resultant mixture through a coolingzone in contact with a heat transfer Wall under conditions of agitationto reduce the temperature thereof below 160 9 F., and finally millingthe resultant grease at a temperature below 160 F. for suflicient timeto eliminate syneresis.

References Cited in the file of this patent UNITED STATES PATENTS2,363,013 Morway et a1 Nov. 21, 1944 2,397,956 Fraser Apr. 9, 19462,433,636 Thurman Dec. 30, 1947 10 Beerbower et a1. Ian. 13, 1948Ashburn et a1. Sept. 28, 1948 Ashburn et a1 Sept. 28, 1928 Puryear et a1Sept. 28, 1948 Puryear et a1 Sept. 28, 1948 Hetherington Feb. 8, 1949McCarthy Jan. 1, 1952 Matthews et a1. Feb. 24, 1953 Jones et a1 Sept.15, 1953

2. THE PROCESS OF PREPARING LITHIUM GREASE COMPRISING FORMING A SLURRYOF LITHIUM STEARATE AND MINERAL OIL CONTAINING SUBTANTIALLY ALL THELITHIUM STEARATE AND APPROXIMATELY HALF THE OIL DESIRED IN THE FINISHEDGREASE, RAISING THE TEMPERATURE OF THE SLURRY TO A TEMPERATURECONSISTENT WITH COMPLETE SOLUTION OF THE LITHIUM STEARATE IN THE OIL ANDABOVE ABOUT 400* F. TO COMPLETELY DISSOLVE THE LITHIUM STEARATE IN THEOIL AND FORM A HOMOGENEOUS SOLUTION FREE FROM UNDISSOLVED LITHIUMSTEARATE PARTICLES BY PASSING THE SLURRY IN A THIN LAYER UNDER AGITATIONIN CONTACT WITH A HEAT TRANSFER WALL CONTINUOUSLY MIXING THE REMAINDEROF THE OIL DESIRED IN THE FINISHED GREASE WITH THE HOT SOLUTION UNDERCONDITIONS CONDUCTIVE TO MIXING WITH A MINIMUM OF SHEAR TO PROVIDEINTIMATE MIXING WITHOUT MICELLE DISRUPTION, THE ADDED OIL HAVING ATEMPERATURE SUCH AS TO PRODUCE A RESULTANT TEMPERATURE OF THE MIXED OILAND